The present invention relates to a working distance detecting device in an ophthalmic apparatus.
In an ophthalmic apparatus such as a funduscopic camera, illuminating beams are projected on an eye to be examined through an object lens located at a position confronting the eye to be examined, and in case of a funduscopic camera, an arrangement is made such that reflected beams of the projected light from the fundus are guided to a photographic optical system. In not only such funduscopic camera but also other ophthalmic apparatus in which illuminating beams are projected on an eye to be examined through an object lens and a flux of beams reflected from the interior of the eye to be examined is guided to a photographic optical system or an observation optical system, there is a risk of occurrence of a flare or ghost because of intrusion of illuminating beams reflected from the face of the cornea of the eye into the photographic or observation optical system.
As means for overcoming this problem of occurrence of a flare or ghost by a flux of reflected beams of illuminating light, there has heretofore been adopted an arrangement in which an annular slit is formed in an illuminating optical system, an apertured mirror is slantingly disposed along the optical axis of the object lens at a position substantially conjugate to the pupil of an eye to be examined with respect to the object lens, an annular flux of illuminating beams passing through the annular slit is once focussed on an annular reflection surface of the perforated mirror and the flux is then made incident into the eyeball through the pupil of the eye from the object lens. If this arrangement is adopted, occurrence of a flare or ghost due to reflection of illuminating beams on the cornea can be prevented when the perforated mirror is located at a position substantially conjugate to the pupil of the eye with respect to the object lens. More specifically, since the distance between the object lens and the perforated mirror is always constant, when the distance between the object lens and the eye, that is, the working distance, is appropriate, occurrence of a flare or ghost can be prevented.
Accordingly, in an ophthalmic apparatus of this type, it is very important to maintain an appropriate working distance while the apparatus is used. This appropriate working distance has heretofore been obtained by delicately moving the apparatus by an operator while observing a picture image produced in an observation optical system, that is, a picture image produced on a Braun tube of a monitor television when illuminating beams are infrared rays or a picture image seen through an eye when illuminating beams are visible rays. When this conventional method is adopted, since not only this operation of adjusting the working distance but also other complicated operations such as the operation of adjustment of alignment of the optical axis of the object lens with the eye to be examined and the focussing operation should simultaneously be performed, a minute flare or ghost is apt to be overlooked. Furthermore, in an ophthalmic apparatus of the type where the operation of alignment adjustment and the focussing operation are carried out by using infrared rays, it is very difficult to detect rays diffused on the cornea or the crystalline lens on a Braun tube of a television.
As means for eliminating this disadvantage, a proposal has been made by Japanese Pat. application No. 81631/79. According to this proposal, a light-receiving portion is disposed in the peripheral region of an object lens and the working distance between the object lens and the face of the cornea of an eye to be examined is detected according to the position of the flux of beams reflected from the cornea on this light-receiving portion. The light receiving portion may be constructed, for example, by a light-receiving plate arranged coaxially with the object lens in the peripheral region of the object lens. In this case, when the radius of the inner circumference of an image of illuminating beams reflected from the cornea, which is formed on the surface of the light-receiving plate, has a predetermined value, it is judged that the working distance is appropriate. Furthermore, the state of alignment of the object lens with an eye to be examined can be known by examining whether or not the inner circumference of the image formed on the surface of the light-receiving plate is coaxial with the optical axis of the object lens.
This working distance detecting device is very advantageous over the conventional ones because the working distance and alignment can be detected very conveniently. Furthermore, if an optical fiber or photoelectric element is used instead of the light-receiving plate, necessary display can easily be accomplished on an image forming portion of the observation optical system. However, in this device, since both the working distance and alignment are detected only by detecting the position of the inner circumference of the annular slit image formed by beams reflected from the cornea of the eye being examined, actual adjustment involves some problems or difficulties. Furthermore, for example, when the field angle of the object lens is larger than 45.degree., occurrence of a flare or ghost due to overlapped passage of the flux of illuminating beams and the flux of beams reflected from the fundus on the front face of the crystalline lens and the cornea is prevented by arranging annular slits at conjugate positions on the front face and the cornea, respectively. In this case, the flux of illuminating beams in the vicinity of the front face of the crystalline lens conjugate to the light-receiving surface with respect to the cornea has a cylindrical shape in which changes of the diameters of the inner and outer circumferences are small. Accordingly, the change of the radius of the inner circumference on the surface detecting the flux of illuminating beams reflected from the cornea is small as compared with the change of the operation distance, and therefore, precise measurement of the operation distance is practically very difficult.